This invention pertains to a device whose principal function is to maintain the performance margin of gas turbine engines at higher levels over considerably longer periods of time, compared to identical models of power plants which do not have the apparatus installed. The device is particularly adapted for use on aircraft gas turbine engines, although it may have other applications on industrial and marine derivatives.
The performance of a gas turbine engine is related to the maintenance of a close fit between the rotating and stationary seals located in the compressor and turbine sections of the engine. The efficiency of the engine in part depends upon the amount of work output that can be extracted from the hot high pressure gas passing through the turbine section of the engine.
Inevitably some gas will avoid the turbine blades by passing through a small gap between the rotating and stationary seals in the turbine section. Engine efficiency can be maintained by minimizing the amount of gas that does bypass the turbine blades. This minimization is accomplished by maintaining the tolerance of the seals as closely as possible.
Efficiency can similarly be maintained by keeping the seals in the compressor sections of the engine at minimal tolerances.
Close tolerances in all of these seals result in higher power output and improved fuel combustion efficiency. In aircraft flight operational terms, the prolonged efficiency resulting from close tolerances assures shorter take off distances, improved time to climb, and greater range and maneuverability. For a large modern four engine commercial airliner, a one percent improvement in fuel consumption saves an average of approximately 100,000 gallons of fuel per year.
That absolute minimal clearances between the rotating and stationary seals are of critical importance to obtain maximum performance with the least fuel burn is borne out by the newer designs of aircraft gas turbine engines which are now fitted with a device which in some cases is called "active tip clearance control". This control system operates by circulating cooled air through passageways and ducts around the turbine case after the aircraft has reached its cruise mode. The cooling contracts the case so as to reduce its diameter and provides a much closer clearance between the turbine blade tips and the stationary seals. Throttle advancement shuts off the cooling air and allows the turbine case to expand as a result of thermal expansion in order to minimize any seal rub resulting from rapid thermal expansion of the rotating elements.
At rated power, large aircraft gas turbine engines normally operate in the temperature range of approximately 600.degree. F. at the compressor discharge point and approximately 2300.degree. F. at the turbine inlet area, with the temperature decreasing toward the turbine outlet.
Once the engine is shut down, the various components of the engine cool down at different rates because of their respective geometries, mass, and locations within the engine. Because hot air rises, the lower sections of the engine cool, and thus contract in length, at a rate greater than that of the components on the upper section of the engine. These differences in cooling rates result in the engine taking an undesirable deformation after the cool down time is completed. This deformation is known in the trade as "engine bow" or "going banana shape". It occurs in all aircraft gas turbine engines to a greater or lesser degree, depending on their design and operating parameters. The "bowing" is most pronounced in the casing and stationary seals of the engine. There may, however, be some bowing in the turbine shaft itself, perhaps to a lesser degree.
This "bowing" is not so pronounced as to be visible by the naked eye. Rather, the contraction and bowing is measured in fractions of an inch applicable to both rotating and stationary components. Evidence of "engine bow" has been established by various tests conducted by original engine manufacturers, wherein concrete evidence of seal rub in various clock locations has been seen on new engines after several initial run-in cycles which could only be attributable to lack of concentricity between rotating and stationary components caused by uneven cooling and resultant "engine bow".
Once the engine has developed a "bow", the compressor and turbine rotors will no longer be aligned perfectly concentrically with their corresponding stationary seals. As a result, if the engine is motored over during the starting transient, the rotors will rub and deteriorate the stationary seals in certain clock locations causing an unwanted increase in seal clearance, and hence, a performance loss.
A secondary problem resulting from the "bowing" phenomenon is that during the starting transient, there is also a measurable vibration because of the bowed condition. This mode gradually disappears as the engine warms up. However, it is recognized by aircraft turbine engine designers that any form of vibration excitation is an unwanted characteristic and should be eliminated or held to an absolute minimum. In addition to passenger discomfort, vibrating patterns can influence bearing life along with other parts affected by vibration excitation.
Another problem resulting from "engine bow" or lack of concentricity between the stationary and rotating components in certain types of high pressure ratio aircraft gas turbine engines is the phenomena known as "start-stall". In this case, the turbomachinery experiences an aerodynamic stall mode during the starting transient due mainly to the unwanted swirl effect or eddying of air through the compressor caused by nonuniform radial clearances between the stationary and rotating components which initially were the result of "engine bow" with attendant seal rub.
Large stationary turbines used primarily for electric power generation also have certain problems that can be minimized by rotating them after shutdown. Certain attempts have been made to minimize the problems associated with large stationary power generation turbines. For example:
U.S. Pat. No. 2,617,253 relates to a safety control system for cooling a stationary gas turbine power plant after shutdown. This system rotates the turbine to cool it in order to prevent hot gasses from the heat exchangers from blowing back into the compressor which is not designed to withstand high temperatures. This system employs the starter motor for the post-shutdown turning and includes a pressure sensitive switch adjacent the compressor and a thermostatic switch near the heat exchangers to start and stop the post-shutdown turning.
U.S. Pat. No. 2,129,529 is directed to an elastic fluid turbine turning mechanism that senses shutdown of a large stationary turbine and engages a motor to turn the turbine slowly in order to prevent sagging of the rotor shaft. The mechanism will continue to rotate the turbine indefinitely because the large weight of the turbine will cause sagging of the shaft if it is held in a stationary position.
U.S. Pat. No. 2,252,456 similarly discloses a system for turning a stationary turbine after shutdown for an indefinite period of time. This system uses an overridable clutch, so that if the turbine is started while being manually turned, the clutch engaging the turbine to the turning motor will slip.
The above-described patented systems are designed for large stationary turbines, and are not suitable for use on gas turbine engines.
As set forth above, the phenomenon of "bowing" and the problems associated therewith have been recognized by experts in the aircraft gas turbine industry. It has also been recognized that turning the turbine after shutdown allows the engine to cool more uniformly, thus avoiding the problems discussed above. In fact, testing has been done by manually engaging an auxiliary motor to the engine auxiliary shaft for the purpose of post-shutdown turning. However, no attempt has previously been made to develop a convenient self-contained operator independent system for automatically turning a gas turbine after shutdown.
A post-shutdown turning device for an aircraft turbine engine has unique sensing, size, weight and power requirements that are not relevant to the larger, stationary power generating turbines. In addition, the environment in which the aircraft turbine engines are used periodically subjects them to loading wind gusts that would interfere with post-shutdown turning by a light weight motor. Also, the rugged environment requires a device that can operate under extreme temperatures, vibration, and shock. The reliability of the device must also exceed the time between overhaul period for its benefit to be fully realized.